Devices in which paper, films or other relatively thin sheet-like materials are conveyed, such as printing presses, copying machines and the like, frequently use combinations of rollers pressed against each other between which the paper, film or the like are passed. In general, at least one of the rollers is provided with a resilient outer layer of material such as, for example, rubber.
In the conveyance of paper by means of such roller combinations, it is important to have a uniform constant pressure distribution in the nip between the rollers parallel to the center lines of the rollers. If the pressure near the shaft ends is, for example, higher than the pressure at the center, then the resilient outer layer of material will be compressed more at the ends than at the center. This results in a local increase in the speed of the resilient material and consequently a local increase in the speed with which the paper is conveyed between the rollers. This may cause the paper to be folded, creased or even start to tear. Further, if the paper is introduced asymmetrically between the rollers, then the nonuniform pressure distribution will result in the paper running askew, which, in most cases, is very undesirable.
Others have attempted to design devices for measuring the pressure between the rolls in a nip. Such devices are shown in U.S. Pat. Nos. 3,418,850 and 4,016,756 and in several foreign patent applications: European Patent Application No. 91,089; German Patent Application No. 26,53,556 and French Patent Application No. 2,351,722. Typically, these devices utilize piezoelectric effects such as described in U.S. Pat. No. 4,499,394; Japanese Patent Abstract No. 59-94028, Vol. 8, No. 211 (Sept. 26, 1984) and an article entitled "Piezo polymer promises low-cost robotic sensors" in Electronic Design, Vol. 31, No. 11 (May 1983).
None of these devices, however, provides a system by which the pressure which is exerted on the paper, film or the like during conveyance through a roller combination can be determined locally to optimize the adjustment of the rollers and thereby avoid the undesirable consequences mentioned above.